Pickup tube having a photoconductive target of enlarged crystal structure



May 17, 1966 F. VAN LooN sHALLCROss 3,252,029

PICKUP TUBE HAVING A PHOTOCONDUCTIVE TARGET OF ENLARGED CRYSTAL STRUCTURE Filed July 6, 1962.

mmummvanv.

INVENTOR. Aww( ww aawjf/,auc'aff United States Patent O PICKUP TUBE HAVING A PHOTOCONDUCTIVE TARGET F ENLARGE!) CRYSTAL STRUCTURE Frank Van Loon Shallcross, Princeton Junction, NJ., as-

signor to Radio Corporation of America, a corporation of Delaware Filed `Iuly 6, 1962, Ser. No. 207,993 6 Claims. (Cl. 313-65) the other end of the envelope 'there is a transparent electrically conducting signal plate on which is deposited a layer of photoconductive material. When an image is focussed onto the photoconductive material, a charge is developed on the beam side of the photoconductive material which is then read off by the electron beam to provide output signals from the signal plate of the'tube.

Tubes of the type briefly described above are limited by the gain obtainable from the photoconductor and by the response time of the photoconductor. Thus, the sensitivity of the tube is controlled primarily by the gain obtainable from the layer of photoconductive material. The speed of response is controlled partly 'by the circuits used, and partially by the time which elapses between when the image strikes the photoconductor and when the charge is developed on the beam side of the photoconductor. The speed of response is also affected by the time required for-the beam to remove the charge produced by the light after the light image has been removed.

It is therefore an object of this invention to provide a new and novel pickup or camera tube.

It is another object of this invention to provide an improved camera tube having a novel photoconductive layervcharacterized in its high quantum gain and short response time.

These and other objects are accomplished in accordance with this invention by providing a pickup or camera tube having a recrystallized photoconductive target electrode in which the crystals of photoconductive material are large, i.e., the crystallites are at least as large as the film thickness. The crystallite size is preferably selected so that grain boundaries cannot be -resolved by the beam. The visibility of the grain boundaries in the transmitted picture can also be avoided by forming area of a single crystal.

The invention will be more clearly understood by reference to the accompanying single sheet of drawings wherein:

FIG. 1 is a sectional view of a pickup tube made in accordance with this invention;

FIG. 2 is an enlarged sectional View of a stage of manufacturing, in FIG. 1; and,

FIG. 3 is an enlarged sectional view of a target shown in FIG. l.

Referringnow to FIG. 1 in detail, there is shown a photoconductive type pickup tube 10. The pickup tube is an example of Aa tube wherein this invention is particularly useful, and the invention will be explained in detail in connection with this type of tube. The tube 10 comprises an evacuated envelope 12 having an electron gun assembly 14 positioned in one end thereof for producing an electron beam. By means of potentials applied to the electron gun 14 and to an -accelerating electrode 16,

`the entirev target the target during. for use in the tube illustrated A electrode 20 deposited on a transparent face plate 22.

The face plate 22 forms `an end `of the envelope 12. The transparent electrically conductive electrode 20 is made of a material that is selected for its transparency to radiations of the particular wavelengths of interest an-d for its electrical conductivity. For the visible range of wavelengths, a layer of tin oxide is conventionally used. The transparent conductive coating 20 is deposited in' contact with'an electrically conductive sealing ring '24 so that, during operation of the device 10, an electrical potential may be applied to ythe transparent conductive electrode 20 and the transparent conductive electrode may function as a signal plate for obtaining output signals from the device 10.

On the transparent conductive electrode 20 there isv deposited a layer of photoconductive material 26. The photoconductive material 26 comprises large area crystals rather than microcrystalline or amorphous films. The large area crystals are used because of the high quantum gains obtainable from such a film and the short response times that have been observed in such films. Specifically, the photoconductive material 26 is single-crystalline, i.e., one crystal is used Which is at least the size of the scanned raster, to a maximum equal'to the vidicon target shown in FIG. 1, or polycrystalline with the crystallites being at least as large as the lilm thickness. [Film thickness within the approximate range of two to ten microns Iare conventionally used. Thus, targets for the tube of FIG. 1 are primarily composed of crystallites varying in size from a minimum size having at least one dimension that is at least as large as the iilm thickness to a maximum size of one crystal per tube. Within this range, the crystallite size is preferably selected so that the grain boundaries cannot be resolved by the beam. Thus, a single crystal is a preferred size since resolved by the beam. Also, crystallites of a size approximately equal to the beam resolving power, i.e. conventionally ll0 to 100 microns, cannot be resolved by the beam and are also a preferred size.

The deposition of the photoconductive'layer 26, illustrated in FIG. 2, is by means of a recrystallized photoconductive layer. By recrystallization is meant the process wherein a photoconductive material is deposited, usually by evaporation, Vonto the target. The deposited layer may or may not include crystals. Subsequently, the deposited layer s processed to form large crystals. As was previously stated the crystals so formed by the recrystallization process have at least one dimension that is at least as large as the lm thickness. For example, a polycrystalline cadmium sulfide iilm has been coated on a tin oxide electrically conducting electrode 20 by vacuum evaporating cadmium sulde material on top of the electr'ode 20 with a substrate temperature of about 170 C. The impurities present will mainly be lost during the evaporation process. Therefore, impurity levels of a few hundred parts per million may be used. The evaporation is continued until cadmium sulde material is deposited to a thickness of about 4 microns. The cadmium sulfide lilm was then baked at atmospheric pressure in an atmosphere of air and sulfur vapor with a sulfur Patented May 17, l966 dimension substantially.

the grain boundaries would not be.

vapor pressure of approximately `one to ten mm. Hg, and at a temperature ofr250 C. to 260 C. for approximately three hours. A layer of silver 27 was then evaporated onto the photoconductive material (the cadmium sulfide) to a thickness of approximately 120 Angstrom units. The target 1S was then heated in argon at atmospheric pressure .to a temperature of approximately 520 C. for a period of time of approximately one hour to recrysta=l- 'lize the cadmium sulfide. On subsequent examination of a lm, manufactured as Vdescribed above, it was seen that the photoconductor was recrystallized with crystallites of approximately 1 mm. dimensions.

Another method of forming a recrystallized cadmium y sulfide layer is by first evaporating a layer of silver, then the cadmium sulfide material rather than the opposite order as was described above. This latter method will minimize the problem of ahigh surface conductivity layer, or spots of silver, affecting resolution of the stored image in the completed target. Y

Materials other than silver, examples of which are copper and indium or mixtures thereof, may also be used to promote recrystal'lization of cadmium sulfide and may be deposited either under or over the cadmium suliide material.

Other photoconductive materials may also be prepared in the form of single crystals, or large crystals, having crystallite sizes that have at least one dimension that is at least as largeV as the lilm thickness. Known photoconductive materials which are believed to be capable of being formed in this crystal size by a recrystallization process include cadmium selenide, zinc selenide, zinc sulfide, arsenic trisulde, antimony trisuliide, cadmiumindium sulfide, and zinc-indium sulfide.

Inv FIG. 3 there is illustrated schematically a sectional view of a recrystallized layer of photoconductive material 26 deposited on the transparent signal plate 20. As illustrated there are a plurality of crystallites included in the target 18. The target area is substantially covered by crystallites which have at least one dimension that is within the range of being at least as large as the film thickness up to a maximum -size of a single crystal over the complete target. As was previously stated, within this range, the crystallites are preferably of such a size that the grain boundaries cannot be resolved by the beam.

The very high gain materials described in this invention may also be prepared having lower resistivities than those used in the prior art tubes. Thus, depending upon the material selected and its intended use, these materials may have a sufficiently high gain that conventional charge storage is not necessary for operation. Thus, in this latter `type of operation those photons striking a given elemental area during the interval between scans increase the conductivity of the photoconductor in this area, thus alecting ythe size of the current pulse which tiows in the pho-toconductor at the instant of scanning. As is obvious, extremely high gains would be necessary I, in this mode of operation since some photoexcited charge is lost by recombination between scans.

Thus, this invention describes a recrystallized lilm of photoconductive material in which the crystallites are at least as large as the ilm thickness. By means of this invention, much higher quantum gains with short respouse time may be obtained which are much greater than those obtainable with conventional photoconductors now used in the vidicon. The use of such materials in the vidicon are believed to provide higher sensitivity with improved response times in the charge-storage mode of operation as well as permitting other types of operation not requiring charge storage. It should be clearly understood that the large-crystalline lilms may be made sensitive to visible light as well as X-ray, ultra-violet or infrared radiation. Any desired exciting radiations may be used by selecting the spectral sensitivity range of the photoconductive materials.

I claim:

1. A pickup tube comprising an evacuated envelope:

(a) an electron gun in one end of said envelope for producing an electron beam;

(b) a target electrode in said envelope and in the path of said electron beam;

(c) said target Velectrode comprising a transparent electrode having a recrystallized layer of 'photoconductive material thereon; Y Y

(d) said recrystallized layer of photoconductive material comprising at least one crystal of photoconductor; and,

(e) said at least one crystal having at least one dimensionV that is at least as large as the thickness Vof said layer of photoconductive material.

2. A pickup tube as in claim 1 wherein said at least one dimension of said at least one crystal is substantially as large in width as said transparent electrode.

3. A pickup tube comprising an evacuated envelope:

(a) an electron gun in one end of said'envelope for producing an electron beam;

(b) a target electrode in said envelope and in the path of said electron beam;

(c) means for scanning said beam in a raster over said target electrode;

(d) said target electrode comprising a transparent electrode having a recrystallized layer of photoconductive material thereon;

(e). said recrystallized layer of photoconductive material comprising at least one crystal of photoconductor; and,

(t) said at least one crystal being at least as large as said raster.

4. A camera .tube comprising an evacuated envelope:

(a) an electron gun in one end of said envelope for producing an electron beam; i

(b) a target .electrode in said envelope and in the path of said electron beam;

(c) said` target electrode comprising a transparent electrode having a recrystallized layer of photoconductive material thereon;

(d) said recrystallized layer of photoconductive inaterial comprising a plurality of crystals of photo conductor;

(e) each of said crystals being at least las large as the thickness of said layer of photoconductive material; and, A

(f) each of said crystals being substantially as large as the diameter of said beam.

5. A pickuptube comprising an evacuated envelope:

(a) an electron gun in one end of said envelope for producing an electron beam;

(b) a target electrode in said path of said electron beam;

(c) said target electrode comprising a transparent electrode having at least one recrystallized layer of photoconductive material thereon;

(d) said recrystallized layer of photoconductive material comprising at least one crystal of photoconductor;

(e) said at least one crystal of photoconductor having at least one dimensionat least as large as the thickness of said layer of photoconductive material; and,

(t) said photoconductive material being selected from ,the group consisting of cadmium sullide, cadmium selenide, zinc selenide, zinc sulfide, arsenic trisulide, antimony trisulde, cadmium-indium sulfide, and zinc-indium sulfide.

6. A pickup tubel comprising an evacuated envelope:

(a)an electron gun. in one end of said envelope for producing an electron beam;

(b) a target electrode in said envelope and in the path 0f said electron beam;

envelope and in the (c) said target electrode comprising a transparent electrode having a recrystallized layer of cadmium sulfide photoconductive material thereon;

(d) said recrystallized layer of cadmium sulde photoconductive material comprising at least one crystal of cadmium sulde; and,

(e) said at least one crystal of cadmium sulfide having at least one dimension that is at least as large as the thickness of said layer of cadmiumsulde photoconductive material.

6 References Cited by the Examiner UNITED STATES PATENTS 3,106,488 10/1963 Lubszynski 313-65 X ROBERT SEGAL, Primary Examiner. DAVID J. GALVIN, Examiner.

V. LAFRANCHI, Assistant Examiner. 

1. A PICKUP TUBE COMPRISING AN EVACUATED ENVELOPE: (A) AN ELECTRON GUN IN ONE END OF SAID ENVELOPE FOR PRODUCING AN ELECTRON BEAM; (B) A TARGET ELECTRODE IN SAID ENVELOPE AND IN THE PATH OF SAID ELECTRON BEAM; (C) SAID TARGET ELECTRODE COMPRISING A TRANSPARENT ELECTRODE HAVING A RECRYSTALLIZED LAYER OF PHOTOCONDUCTIVE MATERIAL THEREON; (D) SAID RECRYSTALLIZED LAYER OF PHOTOCONDUCTIVE MATERIAL COMPRISING AT LEAST ONE CRYSTAL OF PHOTOCONDUCTOR; AND, (E) SAID AT LEAST ONE CRYSTAL HAVING AT LEAST ONE DIMENSION THAT IS AT LEAST AS LARGE AS THE THICKNESS OF SAID LAYER OF PHOTOCONDUCTIVE MATERIAL. 